PROJECT SUMMARY/ABSTRACT Urinary tract infections (UTIs) are one of the most commonly diagnosed infections in older adults. UTIs cost $1.6 billion annually, impair health-related quality of life, and can have serious sequellae such as hospitalization, sepsis, or death. At all ages, UTIs are more prevalent in women than men, with up to 50% of all women experiencing a UTI during their lifetime. The incidence of UTI rises in older women with over 10% of women older than 65 and almost 30% of women older than 85 reporting a UTI within the prior 12 months. Among women with UTIs, there exists a subgroup with recurrent UTIs, defined as 3 or more culture proven infections within 12 months, or >2 culture proven infections in a 6 month period. Recurrent UTI is not only more common in women, but especially more common in the post- menopausal life stage. In some women with recurrent UTIs, genetic factors facilitate bacterial adherence and repeated infection. However, recurrent UTI prevalence rises significantly in post- menopausal women, suggesting additional non-genetic mechanisms associated with aging. The urinary microbiome is one potential non-genetic factor that could influence recurrent UTIs with aging. We now have significant evidence that a urinary microbiome exists, and that dysbiosis may be associated with health versus disease. Our long-term goal is to improve our understanding of the microbes that occupy the urinary niche, how these microbes change with aging, and to determine whether particular microbial community types are associated with recurrent UTI. We aim to compare urinary lactobacilli in populations of women without recurrent UTIs to assess how lactobacilli change with aging and with the presence of vaginal estrogen therapy. Next, we aim to assess whether urinary lactobacilli or other microbes are associated with recurrent UTI in postmenopausal women who are using vaginal estrogen. Finally, we aim to determine whether there are distinct microbial community types that are associated with recurrent UTI in older women.